1. Field of the Invention
This invention relates to a method for the formation of diffraction gratings. More particularly, it relates to a method for the formation of diffraction gratings with a periodicity of approximately 2000 .ANG. on the surface of a semiconductor substrate, in which the phase of the diffraction gratings can be changed at will.
2. Description of the Prior Art
A diffraction grating with a periodicity is formed on the surface of a semiconductor crystal, resulting in the reflecting surface of a distributed-feedback (DFB) laser. This distributed-feedback laser is advantageous over other lasers in that cleavage of the crystal is not required for the formation of the resonator, integration can be readily attained, and stabilized single-mode operation can be readily provided. However, as shown in FIG. 1(I), with a distributed-feedback laser in which the diffraction grating 3 composed of grooves 2 with a periodicity on the upper surface of substrate 1, stabilized oscillation in a single mode cannot be achieved. In particular, it is not possible to obtain single-mode operation when high-speed modulation is involved, as when the laser is being used as a light source for optical communication apparatus. In order to solve this problem, a distributed-feedback semiconductor laser such as that shown in FIG. 1(II) has been proposed in recent years in which the grooves 2' of the diffraction grating 3' in the region of the center of the resonator is shifted by half a cycle (namely, the phase of the pattern of the diffraction grating is shifted by .pi.).
The phase shift of the diffraction grating in the middle of the laser resonator is achieved as follows: A photoresist is coated on a semiconductor substrate and exposed by an electronic beam-exposing system and then developed to form the portions corresponding to the grooves of the diffraction grating. The substrate is then etched with an etchant. The remaining photoresist on the substrate functions as an etching mask. However, while the phase of the diffraction grating can be changed at will, to construct even one semiconductor laser is extremely time-consuming; productivity is low, costs are high, and thus the method is not practical. Alternatively, a positive photoresist, in which areas which have been exposed to light are removed by a development process, is coated on the right half of the laser resonator, and a negative photoresist, in which areas which have not been exposed to light are removed by a development process, is coated on the left half of the resonator. Then, both the photoresists are exposed by a holographic exposing system and developed, and the remaining photoresist is used as a mask in the succeeding etching process. However, the efficiency of the negative resist is poor, and the application of the resist is difficult. Moreover, the phase shift is limited to only .pi..